1. Field of the Invention
This invention relates to an aircraft aircrew life support apparatus and is particularly concerned with a breathing demand regulator which when used in combination with a G-protection trousers garment inflation pressure control system meets requirements for protection of an aircrew member exposed to G-load and/or high altitude.
2. Description of the Prior Art
The enhanced agility of modern high performance aircraft designs give such aircraft the ability to perform very highly accelerative manoeuvres both at low altitude and at high altitudes, e.g. in excess of 12,000 meters (40,000 ft). To take advantage of this agility an aircrew member flying the aircraft must be protected against G-induced loss of consciousness, known as G-loc, as well as the effect of exposure to high altitude in the event of loss of cabin pressure. In this regard, unless otherwise specified, references to altitude are to be understood as references to the altitude equivalent to the pressure within an enclosure or cabin within which an aircrew member is situated and which is usually pressurised in relation to the external ambient pressure with the consequence that "cabin altitude" is related to but usually less than the actual altitude of the aircraft.
The partial pressure of oxygen in air decreases with increasing altitude (decreasing total pressure) so that the concentration of oxygen in breathing gas supplied to the aircraft aircrew member must be increased with increasing cabin altitude to maintain the oxygen partial pressure above the minimum value necessary for it to be able to diffuse through the lung tissue and pass to the haemoglobin or red corpuscles in the blood. If, at aircraft operating altitudes above 12000 meters, there is total or partial loss of cabin pressure which causes cabin pressure to fall below 12000 meters equivalent pressure then the overall pressure of the breathing gas delivered to the aircrew member must be increased to a value above cabin ambient pressure so that the minimum critical oxygen pressure is maintained in the lungs, this being referred to as positive pressure breathing (PPB).
Positive pressure breathing at high altitude is aided by exerting pressure around the chest to assist the aircrew member in exhaling used gas from his lungs against the positive pressure in his breathing mask and to enable breathing to be sustained until the aircraft has descended to 12000 meters or below. To meet this requirement the aircrew member wears an inflatable counter-pressure garment ("jerkin") around his chest and back area which is inflated to the same pressure as the pressure in the breathing mask during positive pressure breathing, conveniently by being connected for inflation by breathing gas delivered to the breathing mask.
To counter the effects of high G-load the aircrew member wears an inflatable G-protection trouser garment ("G-suit") which is inflated from a source of high pressure gas, such as engine bleed air. Inflation of the trouser garment may be in response to signals from one or more accelerometers located in the aircraft for sensing accelerative forces, or in response to movement of an inertia mass provided as part of an inflation control valve assembly. When inflated, the trouser garment restricts the flow of blood into the lower extremities of the body where it tends to be forced under the action of the G-load to which the aircrew member is subjected.
It has been found that protection against G-loc is further enhanced by providing positive pressure breathing during periods when high G-loads are being experienced. The increase in breathing pressure causes an approximately equal increase in heart level blood pressure thereby increasing the flow of blood to the brain.
At altitudes which demand positive pressure breathing it is advantageous to inflate the trouser garment to a pressure three to four times that of the pressure in the breathing mask even at times when aircraft flight manoeuvres are not such as to give rise to high G-load. This inflation of the trouser garment counteracts the tendency for blood to be forced into the lower extremities of the body by the high pressure in the lungs and by the counter-pressure garment, which reduces the circulation of blood from the heart to the brain. However, when both altitude and G-load conditions give rise to a requirement for positive pressure breathing, it is considered now that the trouser garment should be inflated to a pressure appropriate to the higher of the prevailing G-load or altitude signals.
Breathing demand regulator and garment inflation pressure control apparatus disclosed in EP-A-0,000,312 and corresponding U.S. Pat. No. 4,230,097 (Intertechnique) has an inertia body movably responsive to acceleration along a predetermined direction for increasing the inflation pressure in a trouser garment when an acceleration in excess of 2G is sensed. The apparatus further includes an aneroid capsule which is responsive to prevailing aircraft cabin pressure for setting a pressure in the trouser garment at a predetermined aircraft operating altitude. As described the apparatus is particularly suited for use with a liquid oxygen breathing system having a converter for supplying gaseous oxygen to the apparatus. The garment inflation pressure control part of the apparatus includes an ejector which is driven by the high pressure gaseous oxygen to induce ambient air into the apparatus for use as garment inflation supply air. This arrangement is wasteful of oxygen and is not suited for use with breathing systems which supply oxygen-enriched breathable gas at lower pressures than liquid oxygen systems.
A further disadvantage of the apparatus as generally disclosed by this reference is to be found in the arrangement of inertia mass and the aneroid capsule. This is such as to isolate the inertia mass signal at altitudes at which the aneroid capsule is effective so that the pressure in the trouser garment is that set by altitude when a higher pressure may be required for protection against G-load.
This disadvantage applies also to a disclosure in the reference for obtaining positive pressure breathing in the presence of G-load and at altitudes in excess of 12,000 meters.
An embodiment disclosed by the reference that does not have the aforementioned disadvantage has the inertia mass and the aneroid capsule arranged in series such that their effect in setting the trouser garment inflation pressure is additive. This does not satisfy the requirement that inflation of the trouser garment must be to a pressure appropriate to the higher of the prevailing G-load or altitude signals.
It is common practice now to provide oxygen-enriched air as breathing gas for an aircrew member of a high performance aircraft from an on-board oxygen generating system (OBOGS) which includes molecular sieve beds comprising zeolite material suited to the retention of nitrogen whilst permitting oxygen to pass through the beds.
A problem with respect to demand valve operation in a breathing regulator suitable for accommodating the lower range of breathing gas pressure available from an OBOGS is overcome by a breathing regulator disclosed in EP-A-0,263,677 (Normalair-Garrett) which provides positive pressure breathing when the cabin altitude exceeds 12,000 meters and, also, when high G-loads are being experienced. Above 12,000 meters cabin altitude, an aneroid valve expands to increasingly restrict the flow of gas from a breathing-pressure control chamber so that pressure in this control chamber increases thereby increasing the pressure of the breathing gas at the regulator outlet to which both breathing mask and counter-pressure garment or jerkin are connected.
When the aircrew member is subjected to high G-loads, i.e. between 3.5 G and 9 G, a further valve regulating outflow from the breathing-pressure control chamber is signalled pneumatically by an anti-G valve to move towards increasingly restricting outflow of gas from the breathing-pressure control chamber so that pressure in that chamber increases to provide (increased) positive pressure breathing in the event that the cabin altitude is below that at which the same degree of positive pressure breathing would be provided. The anti-G valve is an electro-pneumo-mechanical device that controls a supply of inflation air to the G-suit in accordance with sensed G-loads and the signal to the further valve of the demand regulator is obtained by tapping the inflation air line from the anti-G valve to the G-suit.
Whilst individual operation of each of the aneroid valve and the anti-G valve disclosed by EP-A-0,263,677 is satisfactory, the disclosed arrangement of these valves is such that the requirement for positive pressure breathing to be set by the higher of the G-load and altitude signals is not met.